1. Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for compensating a path of a tool used to manufacture a workpiece.
2. Description of the Related Art
Modern centrifugal compressors include multiple stages of impellers mounted axially along a supporting rotor. The centrifugal compressor stage (impeller) may include many blades. The blades in each stage or row are identical to each other, and typically vary in size and shape from stage to stage. FIG. 1 shows a two-dimensional (2D) impeller 10 of a compressor, the impeller having blades 12. FIG. 2 shows another two-dimensional impeller 10 having blades 12 covered by a cover 13. FIGS. 3 and 4 shows three-dimensional (3D) impellers 14 having blades 16. The impeller in FIG. 4 has a shroud 18 covering the blades. The shape of the blades determines whether the impeller is 2D or 3D.
In this regard, it is noted that for a 2D impeller as shown in FIGS. 1 and 2, the machining tool that forms the blades needs to move only up and down (along a Z direct ion) relative to the blades and also in a plane (XY plane) perpendicular to the Z direction in order to be able to form the blades. This type of movement is called 3 axial machining.
However, for a more complex design as the 3D impeller shown in FIGS. 3 and 4, the machining tool needs to move in more directions, as will be discussed later. This is the case as the shroud 18 is not attached to the impeller 14 but is rather an integral part of it. In other words, the impeller 14, blades 16 and shroud 18 are initially part of a single large piece of metal. Using various techniques, parts of the single piece of metal are slowly removed to form the blades and the shroud.
Various manufacturing processes are available for manufacturing the impeller and turbine blades, yet nevertheless the multitude of such blades requires a substantial expenditure of resources and time, which affect the rate of production and cost of the final machine. One such method is Electric Discharge Machining (EDM). EDM is a manufacturing process whereby a wanted shape of an object, called workpiece, is obtained using electrical discharges (sparks). The material removal from the workpiece occurs by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called tool-electrode and is sometimes simply referred to as ‘tool’ or ‘electrode’, whereas the other is called workpiece-electrode, commonly abbreviated as ‘workpiece’. The tool-electrode is moved by the machine around the workpiece, based on the three-axial movement.
When the distance between the two electrodes is reduced, the intensity of the electric field in the volume between the electrodes is expected to become larger than the strength of the dielectric (at least in some point(s)) and therefore the dielectric breaks allowing some current to flow between the two electrodes. This phenomenon is the same as the breakdown of a capacitor (condenser). A collateral effect of this passage of current is that material is removed from both the electrodes.
Once the current flow stops, new liquid dielectric should be conveyed into the inter-electrode volume enabling the removed electrode material solid particles (debris) to be carried away and the insulating proprieties of the dielectric to be restored. This addition of new liquid dielectric in the inter-electrode volume is commonly referred to as flushing. Also, after a current flow, a difference of potential between the two electrodes is restored as it was before the breakdown, so that a new liquid dielectric breakdown can occur.
A second method is Electro Chemical Machining (ECM), which is a method of removing metal by an electrochemical process. ECM is normally used for mass production and is used for working hard materials or materials that are difficult to machine using conventional methods. Its use is limited to electrically conductive materials; however, this includes all metals. ECM can cut small, intricate contours or cavities in extremely hard steel and exotic metals such as titanium, hastelloy, kovar, inconel and carbide.
ECM passes a high current between an electrode and the workpiece, through an electrolyte material removal process having a negatively charged electrode (cathode), a conductive fluid (electrolyte), and a conductive workpiece (anode). The ECM cutting tool is guided along the desired path very close to the work but it does not touch the piece. Unlike EDM however, no sparks are created. High metal removal rates are possible with ECM, along with no thermal or mechanical stresses being transferred to the part, and mirror surface finishes are possible.
The process schematic is such that a cathode (tool) is advanced into an anode (workpiece). The pressurized electrolyte is injected at a set temperature to the area being cut. The feed rate is the exact same rate as the rate of liquefaction of the material. The area in between the tool and the workpiece varies within 0.003 in and 0.030 in.
The EDM and ECM are traditionally used for making the blades in the turbines and compressors. The blades are formed by removing excessive material from an original workpiece, which is a solid piece of metal having no blades. The EDM or ECM electrode carves the desired blades in the solid piece of metal for forming the blades. These blades are typically solid because they are made of a superalloy metal, to resist to the extreme conditions inside the turbine or compressor. However, a limitation that affects one or both EDM and ECM methods is the wear of the electrode used for removing the excessive material. As discussed above, while a current is applied between the electrode of the tool and the workpiece that is desired to be machined, the electric discharge removes material not only from the workpiece but also from the electrode of the tool. Thus, as the electrode is removing more and more material from the workpiece to create the blade, a length of the electrode becomes shorter. The shorter electrode would not be able to remove the desired amount of material from the workpiece for creating the blades.
This shortcoming of the EDM and ECM methods is illustrated by FIG. 5, which shows a theoretical pocket 20, i.e., a desired volume to be removed from the workpiece, and the real pocket 22 that is removed by the above noted methods due to the wear of the electrode or other factors. For forming a single blade, a few pockets around the blade have to be removed. A pocket is a volume of material to be removed in one pass of the tool.
However, for obtaining an accurate blade, it is necessary to compensate the reduction in size of the electrode. One existing method is to move the working electrode from the blade to a fixed location (i.e., a probe on the machine) and to measure the length of the electrode at the fixed location. After measuring the length and determining that the electrode is shorter than it is supposed to be, the operator may adjust the length of the electrode and restart the processing of the blade. Although this method may ensure a proper length of the electrode, the time involved for moving the electrode to the fixed location and measuring and adjusting the electrode is considerable, sometimes up to 40% of the cycle time, which is undesirable. Another approach is to estimate the length of the remaining electrode based on experience. However, this approach is prone to fail when the geometry of the blade is complicated and/or new.
Another approach is described in Chen et al., U.S. Patent Application Publication No. 2006/0138092, the entire content of which is incorporated by reference. Chen et al. discloses that a flat 2D path is selected for each pocket (see Chen's FIG. 3) and each pocket is compensated with a same amount R as shown in Chen's FIG. 6 and explained in paragraphs [0028] to [0036]. However, the method of Chen et al. is not appropriate for a 3D impeller (which is shown in FIGS. 3 and 4) for a couple of reasons.
First, the above discussed methods are limited by the three axial movement of the electrode and blades having complicated geometries as shown in FIGS. 3 and 4 cannot be achieved with this kind of movement.
Second, the complex blades shown in FIGS. 3 and 4 require pockets that are not necessary plane, which cannot be handled by the exiting methods. Third, the existing methods use a same compensation step for all pockets ignoring the change in conditions from pocket to pocket and also the change in other parameters.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks as well as others understood by those skilled in the art after consideration of the subject matter disclosed below.